The standardization body 3GPP (The 3rd Generation Partnership Project) promotes standardization of LTE (Long Term Evolution) as the next generation communication standard of W-CDMA (Wideband Code Division Multiple Access) system (for example, see Non Patent Literatures 1 to 3).
In this LTE, a base station (E-UTRAN NodeB; also referred to as eNB) of a network (E-UTRAN: Evolved Universal Mobile Radio Access Network) has multiple communication cells (also referred to as cells), and a terminal (user equipment; hereinafter also referred to as a UE) belongs to one of the cells. There are two states of the terminal: a state called an idle state (RRC_Idle) in which a radio bearer is not established between the terminal and the base station and a state called a connected state (RRC_Connected) in which a radio bearer is established between the terminal and the base station. When transmitting/receiving data, the terminal is required to transit from the idle state to the connected state.
FIG. 15 is a sequence diagram for illustrating transition from the idle state of a terminal to the connected state. The terminal uses random access means (random access channel procedure; hereinafter also referred to as an RACH procedure) to synchronize with the base station. As shown in FIG. 15, the terminal sends an RACH to the base station, and the base station sends an RACH response message (RACH response) to the terminal as a response to the RACH. Through the above operation, the terminal can synchronize with the base station and can use a signaling radio bearer 0 (hereinafter also referred to as an SRB0) for transmitting/receiving a radio resource control message (hereinafter also referred to as an RRC message) using a common control channel (hereinafter also referred to as a CCCH).
The terminal sends an RRC connection request to the base station to establish an RRC connection, using the SRB0. The base station transmits an RRC connection setup to the terminal using the SRB0 in order to establish a signaling radio bearer 1 (hereinafter referred to as an SRB1) for transmitting/receiving an RRC message and a non-access stratum message (hereinafter also referred to as an NAS message) using a dedicated control channel (hereinafter also referred to as a DCCH). When receiving the RRC connection setup, the terminal establishes the SRB1.
Next, the terminal sends an RRC connection setup complete to the base station using the SRB1 to confirm that establishment of an RRC connection has succeeded and has been completed. The base station sends a security mode command using the SRB1 to enable AS security (access stratum security) using the SRB1. After that, when a security mode complete sent from the terminal is received, AS security is enabled between the terminal and the base station.
At this time, the base station establishes a signaling radio bearer 2 (hereinafter also referred to as an SRB2) for transmitting/receiving an NAS message with a lower priority than the SRB1 in order to prioritize transmission of an RRC message with a higher urgency (for example, a handover command and a measurement report) over an NAS message with a lower urgency (for example, addition of a service). When the base station transmits an RRC connection reconfiguration to the terminal, and the terminal receives the RRC connection reconfiguration, the SRB2 is established. The terminal transmits an RRC connection reconfiguration complete to the base station using the SRB1 in order to confirm that RRC connection reconfiguration has succeeded and has been completed.
This RRC connection reconfiguration includes configuration information about a data radio bearer (hereinafter also referred to as a DRB) for transmitting/receiving data between the terminal and the base station, and the terminal establishes the DRB based on the RRC connection reconfiguration. In this way, the terminal can transit to the connected state.
When the terminal in the connected state moves out of a cell, a technique called handover (hereinafter also referred to as HO) is used in which the terminal switches communication with its own cell to communication with another cell to avoid disconnection of the communication. FIG. 16 is a sequence diagram showing an example of handover. As shown in FIG. 16, the terminal measures received power or received quality on the basis of configuration of measurement of a received signal included in the RRC connection reconfiguration described above. When an event (for example, the received power exceeding a set threshold) causing a measurement report to be sent occurs, the terminal sends a measurement result to a connected base station (hereinafter also referred to as a source eNB) as a measurement report. The source eNB decides a base station to be a handover destination of the terminal (hereinafter also referred to as a target eNB) on the basis of the measurement report, and sends a handover request to the target eNB in order to communicate a request for handover and information required for handover, to the target eNB.
When receiving the handover request, the target eNB sets a handover command which includes measurement configuration, mobility control information, radio resource configuration, security configuration and the like, and sends the handover command to the source eNB as a handover request ACK. When receiving the handover command from the target eNB, the source eNB sends the handover command to the UE without change. At this time, the source eNB sends a DL allocation to the UE. The source eNB transfers the sequence number (hereinafter also referred to as the SN) of a data packet to be sent to the UE earliest, among the SNs of data packets which have not been sent to the UE yet, to the target eNB and transfers data to be sent to the UE also, to the target eNB.
The UE synchronizes with the target eNB using the RACH procedure, sends a handover confirmation to the target eNB, and completes handover. In this way, the UE in the connected state can switch communication from a base station which the UE is communicating with, to another base station without disconnection of the communication.
The measurement configuration for causing the terminal to measure received power or received quality includes information such as measurement identities (MeasID) which are identities indicating measurement, a measurement object (MeasObject) indicating a measurement target, quantity configuration (QuantityConfig) indicating a measurement result filtering processing operation and the like, reporting configuration (ReportConfig) indicating the configuration of a measurement report, quantity configuration indicating the configuration of values of the measurement result, and a measurement gap indicating a period during which data for measuring other frequencies or other systems is neither transmitted nor received. This measurement configuration is included in RRC connection reconfiguration and sent to the UE from the eNB. Among the above, MeasID, MeasObject and ReportConfig perform operations in cooperation with one another. FIG. 17 is a diagram showing an example of the measurement configuration.
As shown in FIG. 17, MeasID is an identity indicating measurement and identifies a measurement configured by combination of MeasObjectID which is an identity indicating MeasObject and ReportConfigID which is an identity indicating ReportConfig. FIG. 18 is a diagram showing an example of MeasObject. As shown in FIG. 18, MeasObject is constituted by a carrier frequency, the bandwidth of measurement, a frequency offset, a list of neighbor cells, a blacklist, a report CGI (cell global identity) and the like. ReportConfig is constituted by the kind of the trigger for a measurement report, a trigger quantity, a report quantity, the maximum number of cells to be reported, a report cycle, the amount of report (reportAmount) and the like.
The ways of sending a measurement report includes: sending a measurement report at the time of occurrence of an event (event trigger reporting), sending it periodically (periodic reporting), and sending it periodically after occurrence of an event (event trigger periodic reporting). There are five kinds of E-UTRAN events, for example, an event of a serving cell being above a threshold, an event of a serving cell being below a threshold, an event of a neighbor cell being better than a servicing cell, an event of a neighbor cell being better than a threshold, and an event of a servicing cell being worse than a threshold 1 and a neighbor cell being better than a threshold 2, and the like.
FIG. 19 is a diagram showing an example of the measurement report. In the example of the measurement report shown in FIG. 19, information of MeasID, reference signal received power (hereinafter also referred to as RSRP) of a serving cell, and reference signal received quality (hereinafter also referred to as RSRQ) of the serving cell is included in the top part, and the next part includes neighbor cell information. The neighbor cell information includes information of a physical cell identity (hereinafter also referred to as a PCI). Furthermore, information of a global cell identity (hereinafter also referred to as a CGI), a tracking area code and a PLMN identity list (public land mobile network identity list; hereinafter also referred to as a PLMN list) is optionally included. This neighbor cell information optionally includes RSRP and RSRQ information. If there are multiple neighbor cells, multiple pieces of neighbor cell information are included. For example, after the first neighbor cell information, the next neighbor cell information is included as shown in FIG. 19. The terminal performs measurement indicated by MeasID and sends a measurement report to the base station. The base station decides whether or not to perform handover on the basis of the measurement report (and, if handover is to be performed, to which cell the handover is to be performed), and, if handover is to be performed, starts a procedure therefore.
Recently, the standardization body 3GPP has promoted standardization of LTE-A (LTE-Advanced) as a next generation radio communication standard compatible with LTE. For LTE-A, introduction of band aggregation (also referred to as carrier aggregation) in which a terminal uses multiple carrier frequencies of one base station is examined. FIG. 20 is a diagram showing the outline of band aggregation. In FIG. 20, an example is shown in which a terminal uses, for example, two component carriers the carrier frequencies of which are f1 and f2, among three component carriers (the carrier frequencies of which are f1, f2 and f3). By using multiple component carriers as described above, improvement of throughput of communication between a terminal and a base station is expected.
However, in the existing method described above, occurrence of an event causing transmission of a measurement report is determined by comparison with a terminal's own cell. Therefore, if multiple frequencies (for example, the two frequencies f1 and f2) are used in band aggregation, the case is just like the case where there are two terminal's own cells. Then, if an event causing transmission of a measurement report occurs in one of the terminal's own cells, the terminal sends a measurement report to a base station, and the base station decides handover on the basis of the measurement report, then appropriate handover is not performed because the other of the terminal's own cell is not considered at all.
Accordingly, it is conceivable to adopt a method in which the base station requests the terminal to transmit a measurement report on the basis of the other of the terminal's own cells. In this case, operations of transmitting RRC connection reconfiguration from the base station and receiving the measurement report of the other of the terminal's own cells from the terminal are required before the base station receives the measurement report of the other of the terminal's own cells, and therefore, it takes prolonged time to perform handover (contrary to the demand for shortening time required for handover as much as possible).